The invention concerns a method to produce a printing form for rotogravure (in particular for heliorotogravure) in which cups are provided in the surface of the printing form, a printing form for rotogravure (in particular for heliorotogravure), and the use of such a printing form in a print device.
Printing forms for rotogravure, also called print cylinders or engraving cylinders, are predominantly produced in the engraving devices by means of a recording unit in the form of a mechanical engraving unit or by means of an electron beam, laser beam or etching.
A sample to be reproduced is scanned with a scanning organ pixel by pixel and line by line in order to acquire an image signal which represents the tone values of the scanned sample. The image signal is corrected according to the requirements of the reproduction, for example according to a predetermined gradation curve, and superimposed with a raster signal to generate the print raster. The recording signal formed via the superimposition of image signal and raster signal controls the recording unit, which moves along in an axial direction on the rotating print cylinder and engraves a series of depressions or recesses (arranged in the print raster) called cups into the generated surface of the print cylinder. The scanning of the sample (which follows the previously specified principle) occurs nowadays as a rule only with electronic scanning of the sample. The image data supplied by the scanning are given to a computer in which a program-aided treatment and processing occur. In many cases, samples nowadays must no longer be scanned because photographs already exist in many cases as digital data, and text and graphics can likewise be generated on the computer in the form of digital data. The computer then supplies the image signal, based on which of the cups were provided in the generated surface of the print cylinder either mechanically or by means of laser direct engraving or a laser mask method. The depths or volumes of the engraved cups determine the tone value to be printed between “black” and “white”, also designated in print technology terminology as “depth” and “light”.
For the print process, the engraved print cylinder is then clamped in a rotogravure rotation machine.
Before the printing event, each cup accepts a quantity of printing ink, dependent on its volume, that corresponds to the tone value to be printed. In the printing event, the ink transfer then occurs from the cups to the printing material.
A rotogravure cylinder customary in practice is generally comprised of a cylinder core that can comprise steel, aluminum, and of late also a synthetic composite, and that is additionally provided with a coating (base layer), for example made of copper. The cups are engraved into this copper coating or into a further specially applied layer. Due to its physical and chemical properties, copper exhibits good engraving properties which aid the generation of high-quality printings. The thickness of the galvanically applied copper engraving surface is approximately 60 μm to 140 μm. Moreover, the copper layer to be engraved is polished, such that the surface is provided with a defined micro-roughness. The information to be printed, made up of image and text, is subsequently applied by means of a diamond engraving tool to the copper surface in the form of a fine cup raster.
However, what is disadvantageous in the use of copper as an engraving material is that it exhibits a relatively low hardness and abrasion resistance. Due to the mechanical requirements in the print process, with increasing operation period wear would occur on the copper layer via the scrapers, which reduces the print quality as well as the service life of the print cylinder and thus limits the circulation quantities. In order to improve the wear resistance of the engraved copper layer, and thus to increase the service life of the print cylinder, it is common in practice to degrease the engraved copper layer before the printing and subsequently to provide it with a layer made of a harder metal (with regard to copper), for example made of chromium, which for example can occur via a galvanization event. Before the finished printing form is inserted into the printing machine, the surface of the applied layer is polished.
After the printing, the layer, as well as the sub-adjacent copper layer comprising the engraving, is chemically, electrochemically, or mechanically removed. The print cylinder is thereby available for a new cycle to produce a further printing form.
Moreover, in rotogravure in the past, printing forms were produced by means of chemical and/or electrolytic etching, which has led to good results. The print cylinder was hereby coated with a masking layer, whereby a photographic exposure of the mask via film samples, the rinsing of the mask, and the etching of the copper surface with, for example, iron chloride subsequently occur.
What was disadvantageous was the low process safety and the insufficiently good depiction of halftones for images. The etching method was further modified in that on the one hand what is known as a photoresist, and on the other hand what is known as a thermoresist, were selected for the mask layer. In both cases, the mask layer was exposed (one also says illustrated) by a laser beam. In the case of the photoresist, the laser beam generates a photochemical conversion of the irradiated places of the resist layer, whereby a developing step is necessary before the etching to generate the finished mask. In the case of the thermal resist, the laser beam generates the finished mask in a step, in that the laser removes the mask layer via thermal processing where a cup should exist via etching. Both methods are complex in the sense that they comprise relatively many process steps. They are therefore susceptible in practice to quality faults. Moreover, they also have the fundamental disadvantage of all etching methods, namely that the halftones are poorly depicted for images.
Furthermore, it is known for generation of the cups on a print cylinder to use the electron beam engraving method applied in materials processing, that has shown very good results due to the high energy of the electron beam and the enormous precision with regard to the beam deflection and beam geometry. The cups are fired at high speed into the copper layer with an electron beam of higher power density. Due to the large expenditure and the high investment costs for an electron beam engraving machine, electron beam engraving was not previously used in practice for the engraving of copper cylinders for rotogravure, but rather only in the steel industry for surface engraving of what are known as texture rollers for the production of sheets, with which textures are rolled into the sheets.
Furthermore, it was attempted to use lasers for rotogravure engraving in order to engrave the print cylinder with an outer copper layer by means of a laser. However, since copper is a very good reflector of laser radiation, very large capacities, and in particular very high power densities are required of the laser to be used in order to melt the copper. In order to solve this problem, it was proposed to replace the copper layer that comprises the engraving with a zinc layer. The cups are fired into a zinc layer with a laser beam. The laser beam engraving of zinc requires overall less beam power than with copper. A substantial disadvantage of this method exists in that the galvanic application of zinc onto a rotogravure cylinder can be less reliable in industrial practice than when the layer comprising the engraving is copper.
As in the engraving of copper, the zinc layer must also be provided after the illustration (laser engraving) with a wear-resistant layer, for example made of chromium, in order to achieve a sufficient service life in the printing machine. The problem thereby exists that the application of chromium onto zinc functions as unreliably as the application of chromium onto copper, such that the combination of a zinc electroplating with a chromium electroplating is complicated. It is therefore necessary to implement further method steps. In addition to the difficult handling of zinc, disposal, in particular in combination with chromium, represents a further problem.